Electrochemical reduction of benzene using a carbon anode



United States Patent 3,488,266 ELECTROCHEMICAL REDUCTION OF BENZENE USING A CARBON ANODE Eddie C. French, Aubrey, Tex., assignor to Continental Oil Company, Ponca City, Okla.

No Drawing. Filed Dec. 12, 1967, Ser. No. 689,784 Int. Cl. B01k 1/00; C07b 29/06 US. Cl. 204-59 9 Claims ABSTRACT OF THE DISCLOSURE A process for electrochemically reducing benzene to 1,4-cyclohexadiene in the presence of liquid ammonia, an alkali metal chloride, a carbon or graphite anode, and a controlled amount of an alcohol, all in the absence of iron and under anhydrous conditions.

DISCLOSURE This invention relates to a method of electrochemically reducing benzene to selectively produce 1,4-cyclohexadiene.

Benzene can be reduced chemically to a mixture of products, such as 1,3 and 1,4 cyclohexadienes, cyclohexene, and cyclohexane in anhydrous low molecular weight amines with metallic lithium or sodium. These alkali metal reductions are of considerable synthetic use, however, the cost of lithium or sodium metal and the amines makes this process too expensive for large scale use. The products formed by this chemical reduction have similar properties and separation into pure components is nearly impossible. Aromatic compounds have been reduced electrolytically. Benzene has been reduced in methylamine while using lithium chloride as a current carrier. All the foregoing methods used platinum as the anode. Platinum is chemically attacked when used as an anode in liquid ammonia with sodium chloride as a current carrier.

It has now been unexpectedly discovered that benzene can be reduced to selectively produce 1,4-cyclohexadiene when said reduction is carried out electrolytically in the presence of liquid ammonia, an anode selected from the group consisting of carbon and graphite, about 3.0 weight percent to about 4.0 weight percent of an alcohol, and a current carrier of sodium chloride. For highest yields it has been discovered that said reaction should be carried out in the absence of iron contamination and under anhydrous conditions. No reduction of benzene occurs when the anode is carbon or graphite unless the amount of alcohol present is controlled within rather narrow limits. At about 3.0 to 4.0 weight percent alcohol concentration the current efficiencies are sufficiently high to allow the reaction to proceed rapidly. At concentrations above and below this amount the current efficiencies are very low.

Table I shows the effect of alcohol concentration.

TABLE I Weight percent Percent current methanol efficiency These data clearly show that good current efiiciencies can only be obtained with a carbon anode if the alcohol concentration is carefully controlled.

An outstanding feature of thisreaction is its selectivity of products. Benzene was converted to 1,4-cyclohexadiene ice with selectivity of greater than 98 percent. The remaining 2 percent was cyclohexene, probably formed by the reduction of the 1,4-cyclohexadiene. If the reaction was carried to near completion (reduction of all the benzene), the amount of cyclohexene would increase; however, the reaction can be carried to 55 percent conversion to 1,4- cyclohexadiene with only 2 percent cyclohexene by-prodnot and still maintain high current efficiencies. The remainder was unreacted benzene.

Current efficiencies vary over a wide range and are dependent on many factors. Efficiencies of 40 percent can be obtained by using the best known conditions and not carrying the reaction past 25 percent conversion. A current efficiency of 30 to 33 percent was obtained where the conversion was 55 percent.

Numerous materials can be used as cathodes. The desirable properties of the cathode are: (1) high hydro gen overvoltages, (2) resistance to attack by anhydrous ammonia, (3) good conductor, (4) readily available, and (5) low cost. Aluminum, zinc, and platinum have been used with aluminum being preferred because of its availability and low cost.

Numerous materials were tested as anodes. Included were platinum, gold, titanium, cast iron, stainless steel, nickel, zirconium, molybdenum, tungsten, antimony, tin, carbon and graphite. Good current efficiencies can be obtained with a platinum anode with 1 percent alcohol present. When the anode is carbon or graphite, good current efficiencies can only be obtained with the alcohol concentration controlled between about 3.0 and 4.0 weight percent.

Methanol, ethanol and isopropanol are among the alcohols which may be used to produce the beneficial effect with the carbon or graphite anode.

The effect of current density is closely related to the cathode potential as shown in Table II.

TABLE II Effect of cathode current density on current efficiency Current density Percent current ma./in. efficiency Sodium formed at the cathode by the reduction of sodium ions present will react rapidly with any water present rather than the benzene. This results in a drop in current efficiency. Also, iron catalyzes the reaction of sodium with ammonia. If iron or other transition metals are present the sodium reacts with the ammonia and no reduction of benzene occurs.

The effect of benzene concentration was not as critical as other variables; however, highest current efficiencies were obtained when the benzene concentration was 15 to 20 percent.

Temperatures near -33 C. were necessary to obtain good current efliciencies. The yield was not affected by the temperature.

The over-all reaction for the reduction of benzene to 1,4-cyclohexadiene is:

a@ GNHa 3( am This is a chemical reaction. The electrode reactions are:

Cathode: 6Na++6e -6Na Anode: 8NH 6e- N +6NH Total electrode reaction:

8NH 6Na+ 6Na N 6NH 3 (Na 3Na- Na The alcohol then acts as an acid in the ammonia, displaces the sodium, and forms the 1,4-cyclohexadiene.

The alcohol is regenerated by the reaction of the sodium alkoxide with ammonium ions which are formed at the anode.

Mass spectroscopy analysis showed the presence of nitrogen and hydrogen in the off-gases. Nitrogen was liberated at the anode from the oxidation of ammonia. Hydrogen was formed by the reaction of sodium with ammonia.

This reaction is catalyzed by the presence of iron and other transition metals. The competition between this reaction and the benzene-sodium reaction thus accounts for the observed 40% current efiiciency.

Separation of the products formed can be accomplished by fractional crystallization or fractional distillation.

For a fuller understanding of the present invention, reference will be had to the following examples.

EXAMPLE I Twenty-five grams of benzene were dissolved in ,150 grams of liquid ammonia along with 8 grams of sodium chloride and 6 grams of methanol. The solution was maintained at -33 C. in an aluminum pressure vessel. The cathode was aluminum and the anode was carbon. Current density was maintained at 100 ma./in. for a period of 20 hours. Benzene was reduced at the cathode to 1,4-cyclohexadiene. No other reduction products were formed during the electrolysis. The current efliciency was 40.5 percent.

4 EXAMPLE II This example was run in exactly the same manner and under the same conditions as Example I except that only 2 grams of methanol were added. The current efiiciency was only 0.7 percent.

EXAMPLE III This example was run in exactly the same manner and under the same conditions as Example I except that 8.6 grams of methanol were added. The current efliciency was only 0.5 percent.

Having thus described the invention by providing specific examples thereof, it is to be understood that no undue limitations or restrictions are to be drawn by reason thereof and that many variations and modifications are within the scope of the invention.

What is claimed is:

1. In a pr0cess for the electrochemical reduction of benzene to 1,4-cyclohexadiene in liquid ammonia with sodium chloride as a current carrier, under substantial absence of iron, the improvement which comprises adding between about 3.0 weight percent and about 4.0 weight percent of an alcohol and using an anode selected from the group consisting of carbon and graphite.

2. The process of claim 1 wherein said alcohol is selected from the group consisting of methanol, ethanol and isopropanol.

3. The proces of claim 2 wherein the anode is carbon.

4. The process of claim 2 wherein the anode is graphite.

5.1 The process of claim 2 wherein the alcohol is methano 6. The process of claim 2 wherein the alcohol is ethanol.

7. The process of claim 2 wherein the alcohol is isopropanol.

8. The process of claim 2 wherein about 3.0 percent alcohol is added.

9. The process of claim 2 wherein about 4.0 percent alcohol is added.

' References Cited UNITED STATES PATENTS 3,361,653 1/1968 Miller 204-59 HOWARD S. WILLIAMS, Primary Examiner 

